It is known to sense pressure changes, and pressure variations within the combustion chamber of an internal combustion engine (ICE) by utilizing an ion current sensor. Operation of the ion current sensor is based on the generation of electrically charged molecules and atoms, that is, ions, as well as free electrons, when hydrocarbons are burned. If a sensing element having two electrodes is positioned within the combustion chamber, and a biasing or measuring voltage is connected across the electrodes, an ion current will flow which depends on the ion concentration. This ion concentration, in turn, changes with pressure in the combustion chamber. The ion current is sensed and will be representative of the combustion event occurring within the combustion chamber.
It has previously been proposed to utilize an ion current signal to recognize knocking operation of an ICE. Such an apparatus is described, for example, in German patent disclosure document DE-OS No. 28 02 202, in which a spark plug is used simultaneously as the ion sensor, to sense pressure variation within the combustion chamber of an ICE. The spark plug, thus, serves simultaneously as the sparking element as well as the ion current sensor, or electrode structure. The spark plug is connected to the ignition circuit in standard, routine manner. The spark voltage applied to the spark plug is substantially higher than the measuring voltage supplied to sense the ion current. The measuring circuit and the ignition circuit are decoupled. In the known structure, a diode, or a high resistance resistor is located within the measuring circuit in order to suppress the effect of the high ignition voltage on the measuring circuit. The system as described operates with ignition systems customary in ICEs, that is, the ignition voltage is negative. Consequently, the ion current measuring circuit has to apply a negative voltage. If this polarity is used in the customary electrode arrangement in spark plugs, the signal in the ion current sensor will be low, so that signal variations, for example representative of knocking of the engine, cannot be reliably recognized under all operation conditions.